I mean every other week. I wasn’t aware of the other interpretation, but I think in combination with “The Sunday thread” it’s unambiguous?

I have never heard fortnightly, but then I’m not a native speaker. Is that commonly used?

You need different Subdomains as you suggested in your first paragraph. And add a reverse proxy like nginx or caddy to the machine which then proxies the different subdomains to the respective services (e.g. lemmy.your.site to localhost:2222, mbin.your.site to localhost:3333).

Theoretically, you could put a landing page behind some SSO/iam like authentik, and then link to the subdomains from the landing page, but eventually users will need be on the subdomain to use a specific site.